1. Field of the Invention
This invention relates generally to improvements in hydraulic methods and apparatus for conducting liquid downward through a gravity-flow subatmospheric-pressure shaft and more particularly concerns the use of an automatically controlled inlet valve assembly and a subatmospheric-pressure shaft to hydraulically conduct liquid by gravity flow from a first elevation to a lower elevation.
A system involving the present invention finds wide application in the field of liquid transport.
2. Description of the Prior Art
A related hydraulic system employing a subatmospheric-pressure shaft for conducting liquid downward by gravity flow is disclosed in U.S. Pat. No. 3,939,066 as part of a method and apparatus for treating solids in sewage material. A study of subatmospheric-pressure vertical shaft flow also was conducted by others at the St. Anthony Falls Hydraulic Laboratory of the University of Minnesota, as reported in "Hydraulics of Long Vertical Conduit and Associated Cavitation", under Contract EPA 14-12861 with the Environmental Protection Agency of the United States Government.
Another system has recently been proposed for conducting overflow sewage material from a near surface sewer system to a lower or so-called "deep tunnel" system for temporary storage by the Metropolitan Sanitary District of Chicago, Illinois. The deficiencies of known systems involving an atmospheric-pressure gravity-flow shaft are well illustrated by the latter proposed overflow system.
Combined sewers serving metropolitan areas and carrying human and industrial waste and runoff from the precipitation become overloaded during storm periods and previously discharged directly into streams and other waterways without any treatment. In the noted Chicago plan for solving the problem of overflows of combined sewers during runoff, tunnels constructed deep underground in rock strata, e.g., at a depth of 200 or more feet, receive the overflows and conduct them to places of temporary storage at a lower elevation, from which the overflows can be pumped at a later, more convenient time to sewage treatment plants. This plan is markedly less expensive than the use of separate storm and sanitary sewers and much more effective in managing the pollution associated with these overflows. The benefits of such a plan stem in part from the use of the natural structural properties of the pre-existing, sound underground rock for construction of economical tunnels and from the avoidance of nearly all of the above ground disruption that near surface plans entail.
In such a tunnel plan, conduit shafts are used to connect the near surface sewer system to the tunnels. These conduit shafts or pipes generally fall into either of two categories. The first type of shaft employed in such a plan is an atmospheric pressure shaft and is designed to entrain air from the atmosphere as an inherent part of the process of conducting the sewer overflow from the higher to the lower elevation. It will be appreciated that this overflow is highly liquid, being primarily water with a small percentage of solids and/or other liquids in solution or suspension. For convenience, this medium will sometimes simply be referred to as "water" or "liquid" hereinafter.
With an atmospheric pressure shaft, both air and water are conveyed downward. At the bottom of the shaft, the air is separated from the water and is then returned back to the top of the shaft. One proposed type of shaft is generally divided by a slotted partition into two parts, one part for conducting a mixture of air and water to the base of the shaft and then into a pool where the air and water separate, and the other part for venting the air in the pool back up to the top of the shaft. Much of the air induced into the mixture of air and water passing downward in one part of the shaft is entrained in the falling water after passing through the slots in the partition between the two parts of the atmospheric pressure shaft. Such an aerated system requires conduits of adequate size to convey the air downward along with the water, provision of space and/or facilities to separate the air from the water at the lower terminus, and provision for conducting the air back toward the surface. A considerable amount of space and of the cost of construction of an atmospheric pressure shaft is allocated to the movement of air through the system in a cyclical manner. Thus relatively large, complex and costly installations are required.
The second type of shaft for possible employment in the tunnel plan is a subatmospheric pressure shaft which is designed deliberately to exclude entry of air, thus avoiding the problem of handling large volumes of air and permitting a large reduction in shaft size, simplification of the design and reduction in cost. Such a shaft is illustrated in U.S. Pat. No. 3,939,066. Generally, a system employing a subatmospheric pressure shaft involves a reservoir which is fed with liquid, a shaft extending downwardly from the base of the reservoir to a plunge pool, and some form of a control valve in the reservoir which controls the flow of the sewage material from the reservoir into the shaft in a manner to preclude entry of air into the shaft. The plunge pool is designed to dissipate the kinetic energy of the liquid exiting from the shaft. The liquid is then conducted from the plunge pool, e.g., for temporary storage in the underground tunnels and reservoirs in the aforenoted sewage handling system.
In the latter system, liquid enters the inlet reservoir and fills up the volume thereof surrounding the control valve. When the liquid level in the reservoir rises to a sufficient depth to submerge the valve which is seated on the base of the reservoir and blocks the inlet of the shaft and to thereby prevent the entry of air into the subatmospheric pressure shaft when the valve is opened, the valve is lifted from its seat thereby permitting the liquid to flow into the shaft. The liquid then flows into the unvented air space within the shaft and drops down the shaft. The small amount of air contained in the shaft is carried with the liquid toward the bottom of the shaft and vented, and a vacuum is built up within the shaft. As this vacuum increases in strength, it produces an increased pressure differential across the opening of the inlet of the shaft, thus increasing the flow rate into the shaft.
The hydraulic zones of flow with the latter system are as follows: a zone of relatively low velocity of flow of sewage material in the inlet reservoir flowing downward toward the space created by the upward movement of the control valve; a zone of rapid acceleration in the vicinity of this space in which the velocity of the flowing sewage material accelerates to, for example, about fifty feet per second as a result of the combined pressure differential or head of, for example, 10 feet upstream from this space plus the roughly 30 feet of vacuum in the subatmospheric pressure shaft under optimum conditions, making a total pressure head differential across this space on the order of 40 feet; a zone of flow in the upper portion of the shaft which conveys a mixture of sewage material, water vapor, and various gases which have come out of solution in the sewage material as a result of the vacuum condition existing in the shaft; and a zone of flow in the lower portion of the shaft which conveys principally liquid sewage material, with no water vapor, but with some of the gases still entrained as bubbles as the process of redissolution continues. Normal atmospheric pressure creates a back pressure at the lower end of the shaft which is equivalent to about 30 feet of water and which causes the liquid to fill that portion of the conduit, and thereby exclude air from entering the system at the lower end.
The foregoing hydraulic zones of flow exist under equilibrium conditions when the flow is less than the maximum or design flow capacity of the system, as determined by the restriction at the inlet or control valve. Under equilibrium conditions with the flow equal to the maximum design flow capacity of the system, the hydraulic zones of flow are simply those of water (without the water vapor cavity in the upper part of the shaft) flowing under the total head differential represented by the difference in elevation head between the free surface in the inlet reservoir and the piezometric level or pressure in the connecting tunnel. All available elevation head is used for friction loss and velocity head. The inlet would be fully open under these conditions but with the pool in the inlet reservoir maintaining submergence to exclude air.
It will be appreciated that the high velocities and hence high rates of flow attained with the aforedescribed system are dependent upon excluding air from the inlet. This is accomplished by submergence of the intake through control of the inlet valve opening. If no inlet valve were to be used, and if the flow rate of sewage material down the shaft through the subatmospheric pressure shaft is greater than exceeds the flow rate of sewage material entering the reservoir from the sewer system, then the liquid level in the inlet reservoir will fall. If this condition persists, air will be drawn into the shaft. Under such conditions, the pressure within the inlet of the shaft would approach atmospheric, and the flow rate of sewage material down the shaft would decrease dramatically. Then the level of sewage material in the inlet reservoir would again rise until the intake was submerged and entrained air in the shaft was educed by the falling stream and until a vacuum thus was again created in the upper part of the shaft. Then the flow rate of sewage material through the shaft would again increase. This surging or unsteady flow cycle would continue indefinitely, so long as the intake opening permitted flow rates under the vacuum flow condition exceeding the current flow rate into the upper reservoir. Such hydraulic instability is a principal problem of uncontrolled subatmospheric pressure shafts.
Unsteady flow of this type is undesirable, particularly in such large structures as are involved in this invention. Such flow causes vibrations in the various parts of the structure and also surges of flow in the system. It is not considered good engineering practice to tolerate such vibrations and surges in large hydraulic structures, if they can be avoided.
By control of the inlet valve opening, the vacuum flow condition can be maintained in steady state or equilibrium to maintain the desired flow conditions and high flow rates over a wide range of flow rates. The rate at any time should correspond to the rate of inflow to the inlet reservoir with sufficient correspondence to avoid venting of the intake opening at one extreme or overflow at the other. However, it will also be appreciated that in the type of use alluded to above, the inflow rate can vary dramatically. Thus, automatic adjustment of the control valve is desirable. A float control valve is an approach to providing this control. If the increase flow rate of liquid entering the subatmospheric pressure shaft is greater than the flow rate of sewage material entering the reservoir from the sewer system, then the level of liquid in the inlet reservoir falls, and the float control valve lowers, thereby decreasing the intake opening and decreasing the flow rate into the shaft until an equilibrium is again established. Conversely, if the inflow rate increases, the liquid level and the float rise, thereby tending to open the valve to another equilibrium condition. Thus, the float control can control the flow rate entering the shaft in a manner to maintain submergence and prevent venting of air into the shaft, and thus to maintain steady, predictable high rates of flow down the unvented conduit and into the tunnel at the lower level over a wide range of flow rates.
However, a simple float operated valve does not solve the surging problem, but rather may enhance the surging effect. As a vacuum develops in the conduit, a concomitant downward force is exerted on the control valve, and on any related movable structure, tending to offset the effect of the buoyancy of the float control. This would cause the float to move downward to gain additional buoyancy for an equal upward force. Although the additional submergence might require only relatively slight downward movement, the amount of motion is often sufficient to close the initial opening of the control valve, thereby stopping flow of liquid into the conduit. Thus, there would be a series of surges of liquid flow into the shaft, a situation which the system was intended to eliminate.